The invention relates to the reduction of harmonic currents drawn from the AC supply lines by static converters. Current-fed static converter systems consist of a rectifying circuit connected either directly to the AC power lines or to the secondary of an interposed transformer. In high power converter systems an array of rectifier devices, diodes or thyristors, are connected to the multi-phase lines or to multiple secondary windings of polyphase transformers, in order to reduce DC ripple, and curtail harmonic AC line currents. This invention is concerned with the reduction of harmonics present in the currents drawn by the converter system from the AC supply lines.
It is known that the theoretical amplitude of the harmonic currents, expressed as percents of the fundamental current, is: EQU I.sub.h =100/h%
where EQU h=kp.+-.1;
h being the harmonic order number, p the pulse number of the converter and k any positive integer.
For applications allowing moderate harmonic distortion, a six-pulse converter, i.e. having a pulse number p=6, is generally selected. In such case, the harmonics present are those of order h=5, 7, 11, 13, 17, 19, 23, 25, etc. with the corresponding harmonic amplitudes I.sub.h. More demanding harmonic requirements can be met with converters having a pulse number p=12, resulting in the cancellation of every other harmonic pair of the six pulse circuits, thus leaving the harmonics of order h=11, 13, 23, 25, etc. present in the AC line currents. The amplitudes of such harmonics, however, remain unchanged.
Higher pulse numbers are also possible, resulting in the cancellation of further harmonics, but the amplitudes of the remaining harmonics are unchanged.
If these amplitudes are objectionable, the circuit designer has two alternatives depending on the degree of harmonic reduction required: filtering or adding commutation reactances. A high degree of reduction (typically, over an order of magnitude) necessitates the use of filter traps, generally tuned to individual harmonic frequencies and mounted across the AC terminals of the converter. A moderate amount of reduction (about 70% at h=11, 13; even more for a higher order) can be acccomplished by increasing the commutation reactances in the converter system. It has been demonstrated that the larger the commutation reactances, the greater the reduction of the amplitudes of all harmonics present relative to the theoretical amplitude I.sub.h.
The most plausible way of increasing the commutation reactances is by way of selecting a converter transformer of high leakage reactance. The degree of harmonic reduction obtainable in this way, however, is limited because of transformer design constraints. Also, the larger the transformer leakage reactances, the larger the possible mismatch between commutation reactances, which can result in non-theoretical low frequency distortion. Since a converter of p pulse number has 2p effective commutation reactances, this approach would substantially increase the likelihood of a mismatch.